Health problems can occur when cows are fed rations that aren't properly balanced or aren't mixed and delivered in a consistent manner. And each time it happens, the problems may get more severe.
When the rumen pH decreases to a level where the rumen doesn't function optimally, it is referred to as subacute acidosis (SARA). This usually occurs when rumen pH is in the 5.2 – 6.0 range.
A recent paper published in the Journal of Dairy Science illustrates what happens when cows have consecutive bouts of SARA. Eight ruminally cannulated cows were assigned to one of two groups: early lactation cows fed 45% forage or mid lactation cows fed 60% forage. All cows were then fed 3 consecutive acidosis challenges every 14 days by offering about 9 pounds of a 50:50 mix of ground barley and wheat free choice.
Rumen pH plummeted immediately after consuming the barley, wheat mix. The early lactation cows fed the higher grain diets were more susceptible to SARA, had longer bouts of SARA and got worse with each successive acidosis challenge. The mid lactation group also showed more severe pH drops with each successive challenge. The rumen pH's bottomed out at 5.19, 5.07, and 4.90 and lasted 12.2, 13.4, and 15.8 hours/day with each successive bout of acidosis.
This study illustrates the importance of a ration that is well balanced for effective fiber. But more importantly, I think it shows that inadequate mixing, inconsistent mixing and delivery, or any feeding management or cow comfort issues that lead to erratic intake or slug feeding can result in more severe SARA, which can have long term effects on health and productivity. This is especially important during the transition period and early lactation when cows are adapting to their diets. Cows don't adjust to variability.
Reference: Journal of Dairy Science, Vol 91: 3554-3567. Sept. 2008.
--Rick Lundquist is an independent nutrition and management consultant based in Duluth, Minn. You can contact him at firstname.lastname@example.org.
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